1. Field of the Invention
The present invention relates to the field of cleaning or brightening compositions, particularly to such compositions for use on metal surfaces, and more particularly to hydrofluoric acid generating compositions which are used in metal surface treating processes.
2. Background of the Art
It is desirable to have most metal surfaces presented as a highly reflective, shiny surface, for either aesthetic benefits or for functional benefits or both. The first methods for polishing or shining the surfaces of metal were physical treatments, where abrasive surfaces or abrasive materials were rubbed against the surface of the metals to render the surface more smooth and therefore more reflective. The durability of the shininess of a metal surface varied from metal to metal because of the oxidative reactivity of the metal, or its resistance to corrosion. It is not commercially feasible to repeatedly mechanically polish surfaces to maintain their brightness, as this tends to be labor intensive, abrades the surface (removing materials and shortening the effective structural life of the article), and is very inefficient on large surfaces such as vehicles (planes, cars, trucks, snowmobiles, boats, personal water vehicles, and the like). To that end, chemical washing and polishing compositions have been developed.
There are many different types of deposits and corrosion which can accumulate on metal surfaces, particularly where those surfaces are on vehicles. These deposits can also vary from location to location and from season to season as different chemicals and environmental conditions contribute to the different depositing or corroding materials. It is therefore necessary to provide a chemical composition in liquid form which has strong corrosion and film removal properties. This can make surface treatment a complex process for a number of different reasons. The cleaning or brightening solutions must be sufficiently strong to remove unwanted materials but not be so strong as to damage the underlying metal surface. Very strong solvents and strong reducing or oxidizing agents may also be potentially dangerous to workers and handlers, so that containing of the solutions prior to application and protection of workers during application is important.
Hydrofluoric acid is an example of a very strong chemical used to brighten metals, especially in brightening aluminum or aluminum alloys. As shown in U.S. Pat. No. 2,687,346 aluminum and aluminum alloy surfaces on aircraft have been polished with hydrofluoric acid or hydrofluoric acid compounds. The composition of this patent particularly describes a composition which resists flowing and therefore reduces streaking of the surface of the metal by combining the hydrofluoric acid or hydrofluoric acid material with a polystyrene sulfonic acid. A sequestering agent my also be included in the composition to maintain in the cleaning solution any aluminum compounds or complexes formed in the cleaning treatment. Organic sequestering agents such as citric acid, tartaric acid, gluconic acid, and glucono delta lactone, and their ammonium salts are described. The hydrofluoric acid compound may be provided conveniently and preferably as ammonium acid fluoride both for the ease in handling the acid salt (as compared to HF itself) and for the additional contribution of the ammonium.
U.S. Pat. No. 5,417,819 describes a method for forming a highly reflective surface on aluminum alloys comprising brightening a surface of an aluminum alloy body and then desmutting the freshly brightened surface in a desmutting bath. The desmutting bath comprising 10-100 volume percent nitric acid, 0.60 volume percent sulfuric acid, 0-50 volume percent water, and at least 15 grams per liter of a source of fluoride or bifluoride, such as ammonium fluoride.
U.S. Pat. No. 2,625,468 describes a method for brightening aluminum and aluminum alloy surfaces in a chemical bath while maintaining the effectiveness of the chemical bath. The brightening bath generally comprises a composition of nitric acid, ammonium and hydrofluoric acid. Aluminum parts are embedded in the solution and new solution of the initial composition of the bath is added in a quantity equal to the rate of removal of materials from the bath.
U.S. Pat. No. 3,326,803 describes a finely divided composition suitable for use in aqueous solution at a concentration of about 2.8 to 9.5 weight percent comprising a hydrolyzable acid fluoride salt (e.g., selected from the group consisting of alkali metal bifluorides, ammonium fluoride, sodium silicofluorides and mixtures thereof), oxalic acid, water-soluble methylcellulose, acid-stable, water-soluble wetting agents (e.g., selected from the group consisting of anionic and non-ionic wetting agents, and urea. The composition must be added to water, preferably stirred, and then applied to the metal surface to be brightened.
U.S. Pat. No. 4,496,466 describes a brightening bath for aluminum derived from a wet-process phosphoric acid comprising a majority amount of ortho-phosphoric acid, a subsidiary amount of nitric acid, trace amounts of SiO2, chromium and copper, trace amounts of fluoride ion sufficient to maintain a phosphorous to fluorine ratio in the range of 35 to 1 to 100 to 1, trace amounts of iron, magnesium, and aluminum, less than 500 parts per million of organic substances oxidizable in the presence of nitric acid, and fume inhibitors.
Because of the toxicity and difficulty in handling hydrofluoric acid compositions, phosphoric acid based compositions have found a high level of use. These phosphoric acid systems, including the ones mentioned above, may have a wide range of additional functional materials present in the baths for various specific or general purposes as shown by U.S. Pat. Nos. 2,729,551; 3,094,489; 3,009,849; 3,119,726; and 4,496,466. Some of these patents also describe combination phosphoric acid and nitric acid systems along with beneficial additives. However, purely phosphoric acid or phosphoric acid/nitric acid systems do not have some of the desirable properties of hydrofluoric acid systems. It would therefore be desirable to find hydrofluoric acid based systems which are easily used and have reduced potential for toxic exposure of the persons applying the brightening solution.
A two-part composition and a method for applying the two parts of the composition or an immediately mixed single part solution to a metal surface to be brightened is described. The two-part composition comprises the materials needed, when diluted and mixed, to provide hydrofluoric acid in solution for use as a brightening composition. When both components are liquids, they may be diluted and mixed as late as in a spray nozzle head and applied to the surface, with the hydrofluoric acid forming immediately, in transit to the surface, and/or on the surface to be brightened. The two components may comprise one liquid and one solid or flowable powder composition or preferably two liquid compositions which can be mixed immediately before, during or immediately after application to a surface to be brightened. One component comprises a stable fluoride providing compound and the other component comprises an acid which when in solution with the fluoride providing compound will generate hydrofluoric acid. The components are preferably mixed on the same day as they are applied, preferably within hours of application, or even within minutes of application to the surface to be cleaned.